It is known that a metal will have an improved mechanical strength, when heat-treated to change its inner structure.
Multinary Al—Si-based alloys, which comprise an Al—Si-based Al alloy as the basic composition containing one or more elements, e.g., Cu or Mg, have been used for products required to have high mechanical strength, e.g., cast or expanded products of Al-based alloys for automobile members, e.g., those around the wheel, because of their favorable properties. For example, they have higher melt fluidity and fill the mold more smoothly than the other alloys, which are very important for the cast or expanded product. Moreover, they scarcely show cracking when cast, can have still improved strength or elongation when combined with another element, and are low in thermal expansion coefficient and high in resistance to wear.
The examples of Al—Si-based alloys incorporating a small quantity of Mg include AC4A, AC4C and AC4CH, wherein the heat treatment effect due to precipitation of the intermediate phase of Mg2Si increases their strength. In particular, AC4C and AC4CH whose Fe content is limited to 0.20% by weight or less to improve toughness are being used as the alloys for vehicle wheels, e.g., those for automobiles.
The Al alloys for expanded materials, e.g., 2000-series alloys containing Cu and 6000-series alloys containing Mg and Si, also have improved strength as a result of precipitation and hardening of the intermediate phase of Mg2Si or Al2Cu.
As discussed above, taking an Al alloy as an example, it can have increased strength is brought by incorporation of another element and the resultant age-precipitation of the intermediate phase. The heat treatment for the age-precipitation comprises the solution treatment and aging treatment. The solution treatment is the heat treatment which dissolves, at an elevated temperature, the non-equilibrium phase precipitation out during the solidification step to form a solid solution, and cools it with water to form the solid solution uniform at normal temperature. The solution treatment is followed by the aging treatment, which keeps the solid solution at relatively low temperature, to precipitate the element out of the solid solution in which it is dissolved and harden it as the intermediate phase. These heat treatment steps improve mechanical properties of the Al alloy.
The solution and aging treatments of an Al alloy have been effected in atmosphere furnaces, e.g., tunnel furnace with air used as the heat medium. These furnaces have disadvantages which make the solution heat treatment difficult at higher temperature, e.g., slow heating rate and a wide temperature fluctuations of around ±5° C.
The solution heat treatment by the conventional atmosphere furnace takes a long time, a total of around 4 hours or more, to heat the work piece to the dissolution temperature, due to slow heating rate, and to hold it at that temperature for more than 3 hours. The conventional atmosphere furnace, e.g., tunnel furnace, needs a large heat-treatment facilities, which inevitably pushes up the initial investment cost, and also needs a large manpower for time-consuming works and a large quantity of heat energy for increasing and keeping temperature, which increases the running cost.
More recently, use of a fluidized-bed furnace is proposed for solution and aging heat treatment of an Al alloy, in Japanese Patent Laid-Open No. 2000-17413, which, however, describes no particular type of fluidized-bed furnace.
The known conventional fluidized-bed has a structure, e.g., shown in FIGS. 5(a), (b) or (c). The fluidized beds shown in FIGS. 5(a) and (b) are of the so-called indirect heating type, wherein cold air A is blown upward from the air chamber 52 below the distributor 50, passing through the fine holes 55 in the distributor 50 to fluidize the particles 54, e.g., sand, over the distributor 50. The fluidized bed vessel 58 shown in FIG. 5(a) is heated by the heating means 59, e.g., heating wires or gas provided around the external periphery, to heat the particles 54 and work piece put in the fluidized bed. The fluidized bed shown in FIG. 5(b) is provided with the radiant tube system 60 inside as the heating means, to heat the particles 54 and work piece put in the fluidized bed.
The above fluidized beds of indirect heating type has disadvantages, e.g., low heating efficiency and temperature distribution between the area around the heating means and other areas.
On the other hand, the fluidized bed shown in FIG. 5(c) is of direct heating type, wherein hot air B is blown upward through the fine holes 55 in the distributor 50, to fluidize the particles 54 and thereby to form the fluidized bed, and, at the same time, to heat the particles 54 and work piece put in the fluidized bed. The fluidized bed directly heated with hot air has an advantage of good temperature distribution within the bed. The conventional fluidized bed needs baffles 56 over the fine holes 55, as shown in FIG. 6, to prevent the particles 54 from falling through the fine holes 55. It also needs the air chamber below the distributor, which tends to increase its size. Its another disadvantage is that the distributor must have an additional strength to support weight of the particles, e.g., sand, which further increases facility size and investment cost.
It is an object of the invention to provide a hot air blowing type fluidized-bed furnace which can solve the problems involved in the conventional fluidized bed. It needs a lower investment cost and smaller space and prevents thermal energy loss, and suitable for a heat treatment furnace for metals, e.g., Al alloy.
It is another object of the invention to provide a heat-treatment furnace and heat-treatment apparatus which are compact and hence need reduced investment cost and space, and, at the same time, thoroughly prevent thermal energy loss and are capable of being fully automatically operated, to reduce the running cost. It is still another object of the present invention to provide a method of heat treatment.